This invention relates to the reproduction of information from a rotatable record medium and, more particularly, to such a method and apparatus wherein the information is recorded in the form of spaced apart pits in substantially concentric circular tracks, the tracks being scanned by a light beam of controllable size.
Information which exhibits a relatively wide bandwidth should be recorded by a record carrier with a relatively high recording density. This, of course, enables a large amount of information to be recorded and subsequently reproduced from the record carrier. As an example, optical recording and reproducing techniques are known to exhibit favorably high recording densities. By using such techniques, video information, which exhibits a relatively wide bandwidth, may be recorded in, for example, spiral or substantially concentric circular tracks on a rotatable record disc. When such video information is recorded in optically reproducible form, such as by recording the video information in the form of optically detectable pits in the substantially circular tracks, a light beam is caused to scan successive tracks wherein the pits serve to modulate the intensity of the beam. This modulated light beam then is detected, either by transmitting the modulated beam or reflecting the modulated beam to a suitable photodetecting device, thereby recovering the recorded video information.
When video information is recorded in the aforementioned manner, the width of each substantially circular track and the lengths of the respective pits are extremely small. This is a desirable feature in order to achieve a high recording density. This also is necessary in order to provide a record disc of a suitably long playing time. As an example, if one frame of video information is recorded in each track, then, for the NTSC system, thirty frames must be reproduced each second, thereby requiring an angular velocity of the disc of 1800 rpm. Thus, track width and pit length must be small to permit a large number of tracks to be recorded on the disc.
One technique for recording video information in the form of pits in concentric tracks employs frequency modulation of the video signal, and the recorded pits represent such frequency modulations. More particularly, a clipping level is established and the frequency modulated video signal is compared to this clipping level. During each cycle, that portion of the frequency modulated video signal which exceeds the clipping level is recorded in the form of a pit whose length is proportional to the duration of the cycle which exceeds the clipping level. Thus, as the frequency component of the video signal changes, the length of the pit changes inversely.
Not only does the length of a pit vary with the frequency component of the frequency modulated signal, but pit-length also is dependent upon the radius of the track in which the pit is recorded. In fact, the radial dependency of the length of the pits is more pronounced than the frequency dependency thereof. To appreciate this, let it be assumed that the same frequency component is recorded in a track whose radius is, for example, 15 cm. and in a track whose radius is, for example, 5 cm. When the pits in both tracks are recorded, the duration of the frequency modulated signal which exceeds the clipping level is identical. However, since the linear speed of the track whose radius is 15 cm. is three times greater than the linear speed of the track whose radius is 5 cm., the length of the pit which is recorded in the track of greater radius is three times as long as the length of the pit which is recorded in the track of smaller radius. This is because the length of a pit is proportional to the product of the duration of the frequency modulated video signal which exceeds the clipping level and the linear velocity of the track.
During signal reproduction, the information which is represented by the recorded pits is recovered by scanning the record disc with a light beam. If the size of the light beam which is incident on the disc, hereinafter the size of the scanning beam spot, remains constant for the scanning of all tracks regardless of the radius of the particular track which is being scanned, then the relationship between the length of the pit and the size of the scanning beam spot will vary. That is, for longer pits which are recorded in the outer tracks, the size of the scanning beam spot may be too small relative to that pit length. However, when that same beam is used to scan the pits in the inner tracks, the size of the scanning beam spot may be too large with respect to the pits which are recorded in those inner tracks. The difficulties which may arise because of this can be explained mathematically. Let it be assumed that the length of a pit recorded in a track whose radius is, for example, 15 cm. is represented as L.sub.1, and the diameter of the scanning beam spot is represented as r. The pit will be detected when the beam first impinges upon the leading edge of the pit and this detected condition will be maintained while the beam overlaps the pit and until the beam no longer impinges upon the trailing edge of the pit. Consequently, the effective detected length of the pit may be expressed as L.sub.1 +2r. A similar expression may be derived when the beam of constant diameter r scans a pit of length L.sub.2 which is recorded in a track whose radius is 5 cm. In this latter case, the effective detected length of the pit may be expressed as L.sub.2 +2r. If each pit is intended to represent the identical information, such as the same frequency component, then it is expected that a pulse produced by the scanning of the longer pit will be of an identical duration as the pulse which is produced in response to the scanning of the shorter pit. This pulse duration may be expressed as the effective length of a pit divided by the linear velocity of the track in which that pit is recorded, that is, t=L/v. When the pit which is recorded in the track whose radius is 15 cm. is scanned, the resultant pulse duration t.sub.1 may be expressed as t.sub.1 =(L.sub.1 +2r)/v.sub.1, where L.sub.1 +2r is the effective pit length and v.sub.1 is the linear velocity of the track. Similarly, the duration of the pulse which is produced when the pit that is recorded in the track whose radius is 5 cm. is scanned may be expressed as t.sub.2 =( L.sub.2 +2r)/v.sub.2, where L.sub.2 +2r is the effective pit length and v.sub.2 is the linear velocity of the track. In this example, since the radius of the outer track is three times as great as the radius of the inner track, then L.sub.1 =3L.sub.2, and v.sub.1 =3v.sub.2. When these expressions are substituted into the equation for t.sub.1, then the pulse duration may be rewritten as t.sub.1 =(3L.sub.2 +2r)/3v.sub.2 =(L.sub.2 +2/3r)/v.sub.2. Contrary to the expectation that the duration for each pulse which is produced in the respective tracks will be equal, it is seen that t.sub.1 &lt;t.sub.2. Since these pulse durations are not equal, the signal reproducing apparatus will erroneously interpret information which is recorded in tracks of different radii, provided the size of the scanning beam spot remains constant. Stated otherwise, if the size of the scanning beam spot is constant, then the change in the length of a pit due to the particular radius of the track in which the pit is recorded will result in a change in the relationship between the beam spot and the pit length. This changing relationship may result in erroneous output signals during the reproducing operation.
The changing relationship between the length of a pit and the size of the scanning beam spot may result in still further difficulties. When a proper relationship exists between the size of the scanning beam spot and the length of the pit, the pit will be detected by a change in the intensity of the light reflected therefrom, such as a reduction in the intensity of the reflected light when the beam spot impinges upon the pit. This change in the intensity of the reflected light is detected and results in the production of an output pulse. However, if the length of a pit is too long relative to the size of the scanning beam spot, then the intensity of the light which is reflected from the surface of the record disc will be reduced when the leading edge of the pit is scanned, but then the intensity returns to its "normal" level when the beam spot scans the bottom of the pit. This is because the bottom of the pit generally is reflective substantially to the same degree as the surface of the disc. However, when the beam spot next impinges upon the trailing edge of the pit, another reduction in the intensity of the reflected light is produced. Consequently, two output pulses are produced when this pit whose length is too long is scanned by the beam spot. If a single pulse is intended to represent a pit, then the production of two pulses will erroneously be interpreted as being representative of two pits. Hence, errors in the reproduced information will result.
In the foregoing example, the size of the scanning beam spot is too small for the length of the pit. However, if the size of the scanning beam spot is selected so as to be proper for the longer pits, that is, those pits which are recorded in the outer tracks, then an incorrect relationship will exist when that scanning beam spot is used to scan the shorter pits which are recorded in the inner tracks. When such shorter pits are scanned by the larger scanning beam spot, a substantial portion of the beam spot impinges upon the surrounding land, or surface of the disc. As a result, the intensity of the light which is reflected therefrom is changed only insignificantly by the scanned pit. This can be appreciated by assuming that the intensity of the light which is reflected from the disc when the scanning beam impinges upon the land or surface thereof is represented as M, and that the intensity of the light which is reflected when the scanning beam impinges upon the pit is represented as P. The modulation factor m may be defined as m=(M-P)/(M+P). If the size of the scanning beam spot is too large relative to the length of the pit being scanned, as aforesaid, then the reflected light intensity P is almost equal to M. Consequently, the modulation factor m is very small. Hence, the signal-to-noise ratio (S/N) for a beam spot which is too large relative to the length of a pit is small, and the resultant output signal which is reproduced by using that beam to scan the disc is degraded. As a further disadvantage, the scanning beam spot may be so large as to impinge upon adjacent pits which are recorded either in the same or adjacent tracks. This introduces crosstalk, interference and further deterioration in the output signal. If the information which is recorded on the record disc represents video signal information, then the resultant video picture which is reproduced by scanning that disc with such a beam spot is less than satisfactory.
Although the length of a pit which is recorded in a given track may vary as a function of the information represented thereby, for example, if the pits are representative of frequency modulated information, pulse width information or the like, the dependency of pit length upon such information is not as significant as the dependency of pit length upon the radius of the particular track in which such pit is recorded. Hence, even though the length of a pit will vary as a function of such information, this variation will not result in such a large change in the relationship between the size of the scanning beam spot and the length of the pit as to cause the aforenoted problems. As a numerical example, if the length of a pit is representative of the frequency component of a frequency modulated signal, such as a frequency modulated video signal, then a typical center frequency of such a frequency modulated signal is about 8 MHz and the maximum frequency deviation of the frequency modulated signal is about 1.7 MHz. Hence, the length of a pit corresponding to a minimum frequency is about 1.2 times the length of a pit corresponding to a maximum frequency; and this pit length deviation is very small relative to the change in the length of a pit which is recorded in the innermost and outermost tracks, the length of the pit recorded in the outermost track being, for example, three times the length of a pit which is recorded in the innermost track. Hence, even though there is some variation in the length of a pit due to the information which is represented thereby, the effects caused by this small variation are negligible when compared to the aforenoted effects which are caused by pit length variation attributed to the radial dependency of such pits.